1. Field of the Invention
The invention relates to magnetic recording and recovery systems and particularly to ternary magnetic recording systems. The invention is also applicable to digital data communication systems.
2. Description of the Prior Art
Digital magnetic recording systems, such as discs, tape, drums and the like, are in wide spread usage wherein digital data is stored by means of recording flux transitions from one magnetic state of the medium to another. In typical binary systems, a binary 1 is denoted by the presence of a flux transition and a binary 0 is denoted by the absence of a transition. Traditionally, the transitions are effected between the two states of magnetic saturation viz: positive saturation and negative saturation.
It has been a continuing desideratum in such systems to increase the density of the data stored on the medium. It is well known, however, that when flux transitions are stored too close together on the medium intersymbol interference results severely distorting the recovered data. Additionally, since the data recovery system is timed by a clock signal derived from the recorded transitions, long runs of binary 0's which do not result in transitions cause the timing clock to drift resulting in anomalous behavior of the system.
These problems have been significantly reduced by the use of recording codes that translate the binary input data into suitable codes which are then recorded on the medium. Constraints are placed on the code with respect to minimum and maximum spacings between transitions so as to reduce intersymbol interference and provide a stable recovery clock while effecting the highest possible data storage density. A detailed discussion of these problems as well as the properties of a number of magnetic recording codes may be found in U.S. Pat. No. 4,323,931 issued Apr. 6, 1982 to George V. Jacoby entitled "Method and Apparatus for Encoding and Recovering Binary Digital Data", and assigned to the present assignee.
A significant improvement in data storage density was effected by the ternary system disclosed in U.S. patent application Ser. No. 260,248, filed May 5, 1981 in the names of George V. Jacoby and Martin Cohn entitled "Ternary Data Encoding System" and assigned to the present assignee. The system of said Ser. No. 260,248 utilized two recording levels, preferably positive and negative magnetic saturation and transitions between the levels to record binary data in a ternary format. The binary data was encoded into a ternary code wherein a ternary 1 code symbol was recorded as a first kind of signal change between magnetic levels, a ternary 2 code symbol as a second kind of signal change between the magnetic levels and ternary 0 by the absence of a signal change between the levels. Preferably, the first kind of signal change was a single transition between the levels denoted as a "singlet". The second kind of signal change was preferably two closely spaced transitions forming a pulse or "doublet". Thus, the preferred embodiment of said Ser. No. 260,248 was based on representing the ternary code symbols 0, 1 and 2 by nulls, singlets and doublets, respectively.
The ternary system of said Ser. No. 260,248 utilized a ternary variation of the recording code disclosed in said U.S. Pat. No. 4,323,931 providing numerous desirable properties with respect to minimum and maximum spacings between ternary symbols, data storage density, code rate and detection window. Said Ser. No. 260,248 disclosed two ternary, fixed length, recording codes, one code converting groups of two binary digits into code groups of three ternary symbols and an alternative code converting groups of four binary digits into code groups comprising six ternary symbols. The system of said Ser. No. 260,248 utilized look ahead principles to effect merging, skipping or shifting so as to achieve the desirable code constraints. The ternary system of said Ser. No. 260,248 is further described in the IEEE Transactions on Magnetics, Volume MAG-17, No. 6, November 1981 in an article on page 3326 entitled "Ternary 3PM Magnetic Recording Code and System" by George V. Jacoby.
The ternary code of said Ser. No. 260,248 that converts four binary digits into six ternary code symbols has the following constraints. All non-zero symbols are separated by a maximum of eight detents or seven zeroes. Ternary 1's are separated by a minimum of three detents or two zeroes and ternary 1's followed by ternary 2's and ternary 2's followed by ternary 1's are also separated by a minimum of three detents or two zeroes. Additionally, consecutive ternary 2's are separated by a minimum of four detents or three zeroes. The term detent refers to a single timing clock interval as discussed in said Ser. No. 260,248 or said U.S. Pat. No. 4,323,931.
Detection techniques for discriminating between recorded singlets and recorded doublets are discussed in said Ser. No. 260,248 as well as in said article by George V. Jacoby entitled "Ternary 3PM Magnetic Recording Code and System". In order to provide robust recovery discrimination between singlets and doublets, it is desirable to use an amplitude discrimination characteristic as well as zero crossing and peak detection criteria. Amplitude may be utilized for this purpose since, all other factors being equal, the amplitude of a recovered singlet is significantly greater than the amplitude of a recovered doublet. Amplitude, however, is an unreliable discrimination criterion because of the large gain variations normally encountered in such magnetic recording systems. For example, the gain of the system changes as the flying height of the read head varies and also is dependent upon characteristics of the medium such as thickness. Instability in the electronic circuitry may also contribute to the unreliability. Automatic gain control (AGC) circuitry may be utilized to stabilize the gain of the system where the AGC would sense the extant amplitudes of either the recovered singlets or the recovered doublets. It was noted in the recording codes of said Ser. No. 260,248 that the distance between singlets and the distance between doublets was unbounded. Thus, an AGC system responsive to either detected singlets or detected doublets may tend to drift in the absence of the appropriate recorded symbols. It was also noted that with respect to sequences of chained doublets, i.e., three or more doublets, each separated from the adjacent doublets by three zeroes, an error in detection would propagate through the entire chain.
It was desirable in the development of the present invention to utilize the detected singlets for AGC amplitude control since recovered singlets may be utilized more reliably for this purpose than recovered doublets. It would therefore be necessary to limit the run of a finite value between recorded singlets. It was also desirable to impose a constraint on the code so as to eliminate the possibility of error propagation caused by chained doublets.
It would be exceedingly difficult, if not impossible, to impose these two constraints on the code and preserve all of the other desirable properties of the prior code disclosed in said Ser. No. 260,248 and in said article by George V. Jacoby utilizing the conventional fixed length look ahead code format or a conventional variable length code format.